EXPLAINING GLOMERULAR PORES WITH FIBER MATRICES - A VISUALIZATION STUDY BASED ON COMPUTER MODELING

The extracellular space of the glomerular capillary wall is occupied b y a complex meshwork of fibrous molecules. Little is understood about how the size, shape, and charge recognition properties of glomerular u ltrafiltration arise from this space-filling fiber matrix. We studied the problem of size recognition by visualizing the void volume accessi ble to hard spheres in computer-generated three-dimensional homogeneou s random fiber matrices. The spatial organization of the void volume f ollowed a complex ''blob-and-throat'' pattern in which circumscribed c avities of free space within the matrix (''blobs'') were joined to adj acent cavities by narrower throats of void space. For sufficiently sma ll solutes, chains of blobs and throats traversed the matrix, providin g pathways for trans-matrix permeation. The matrices showed threshold or gating properties with respect to permeation: solutes whose radius exceeded a critical value, at which a throat on the last connected tra ns-matrix pathway pinched off, could not cross, whereas smaller solute s had nonzero permeability. The thresholds may give the glomerular fib er matrix porelike response properties and explain why pore models hav e been such a useful means of treating glomerular permselectivity.